Silicon is a primary component of most solar cells manufactured today. The essential photovoltaic mechanism for converting solar energy to electrical energy relies on a PN junction that is formed in a silicon wafer. The PN junction may be formed by implanting a P-type impurity into an N-type silicon, or vice-versa.
The solar cell industry has traditionally relied on silicon discarded by the electronics industry as a material source. This has significantly increased the cost of manufacturing a solar cell because the grade of silicon required for an integrated circuit or memory chip, for example, is far purer and hence more expensive than the silicon required for a solar cell. Boron and phosphorus are two impurities whose content is subject to limits in both electronic-grade silicon and solar-grade silicon. Yet, while the permissible quantity of boron in electronic-grade silicon (e-Si) is only about 0.0002 ppma (parts per million atomic), a level of 0.1 to 3 ppma is acceptable in solar-grade silicon (s-Si). Similarly, for phosphorus, e-Si requires a level of 0.0008 ppma or less, while a proportion in the range of 0.1 to 1.0 ppma can be tolerated in s-Si. In short, the solar industry has normally used silicon that contains boron and phosphorus in concentrations that are orders of magnitude lower than necessary.
This disparity has had a marked effect on the cost and economic viability of solar cells. The price of discarded e-Si wafers and off-spec silicon has typically varied between $45 and $55 per kg and has ranged as high as $150 per kg. These prices can be expected to climb as the demand for silicon by the solar cell industry rises and the availability of scrap e-Si declines. Currently, silicon accounts for approximately 25-40% of the cost of a solar cell, and this proportion is likely to increase as other production costs fall. Thus, a reduction in the cost of silicon would very significantly improve the economics of solar cells. For example, it has been estimated that a 40-50% reduction in the silicon cost would make electricity from solar cells competitive with peak residential rates of power supplied by utilities and that further advancements in solar cell technology might even make electricity from solar cells competitive with off-peak utility rates.
In terms of price, metallurgical grade silicon (m-Si) is an attractive alternative to e-Si, since m-Si sells for only about $1.80 per kg, or about 4% of the typical price of e-Si. Unrefined m-Si cannot be used to manufacture solar cells, however, because it contains a number of impurities—in particular, boron and phosphorus—in quantities that are far too high. Metallurgical-grade silicon contains 10-50 ppma boron and 15-50 ppma phosphorus. Comparing those levels with the quantities acceptable in s-Si (given above) reveals that the amounts of boron and phosphorus in m-Si must be reduced by approximately 90% and 99%, respectively, to make it suitable for use in solar cells. The quantities of other impurities, such as aluminum, calcium, chromium, copper and iron, must also be reduced, but generally there are known processes for removing these materials.
Thus, there is a clear and compelling need for a cost-effective process that can remove impurities from metallurgical-grade silicon, particularly boron and phosphorus, to the degree necessary to render the silicon usable in the manufacture of solar cells.